(1) Field of the Invention
This invention relates to an X-ray generating apparatus for a non-destructive X-ray inspecting system or X-ray analyzing system. Particularly, the invention relates to an apparatus having a very small X-ray source sized in the order of microns to obtain fluoroscopic images of a minute object. More particularly, the invention relates to a microfocus X-ray tube.
(2) Description of the Related Art
Conventionally, X-ray generating apparatus of the type noted above are operable according to the following principle. First, electrons (Sa [A]) are emitted from an electron source maintained at a high negative potential (−Sv [V]) in a vacuum, and are accelerated by a potential difference between the electron source and ground potential 0V. Next, the accelerated electrons are converged to a diameter of 20 to 0.1 μm with an electron lens. The converged electron beam collides with a solid target formed of metal (e.g. tungsten or molybdenum), thereby realizing an X-ray source sized in the order of microns. A maximum energy of X-rays generated at this time is Sv [keV], and the X-ray focal size approximately corresponds to the diameter of the converged electron beam.
An especially high-resolution apparatus among these X-ray generating apparatus is an X-ray tube called a transmission microfocus X-ray generating apparatus. The X-ray tube has a target structure including a vacuum window in the form of an X-ray transmission plate of aluminum or beryllium. The vacuum window has a target metal formed in a thickness of 2 to 10 μm on a vacuum side surface thereof. The X-rays generated by an electron beam colliding with the target metal pass through the vacuum window in the direction of the incident electron beam and are utilized in the atmosphere.
In such a transmission X-ray generating apparatus, an inspection object and an X-ray focus are set close to each other by an extent corresponding to the thickness of the vacuum window to enable, geometrically, high magnification X-ray radiography, thereby to obtain fluoroscopic images of high spatial resolution. Such an X-ray tube is used in an inspection apparatus for searching for minute defects in an inspection object. These inspecting operations will sometimes take several hours per object (see Japanese Unexamined Patent Publication No. 2002-25484 and Japanese Unexamined Patent Publication No. 2000-306533, for example).
However the portion of the target where an electron beam collides becomes high temperature and the target material evaporate and wear away, the X-ray tube will cease emitting X-rays in due time. To overcome this inconvenience, it has been proposed, in the case of a reflection type X-ray tube, to form a heat dissipation layer on an internal layer opposite the electron-colliding surface of the target, to restrain a temperature rise of the target by utilizing heat conduction (see Japanese Unexamined Patent Publication No. 2000-082430, for example).
The conventional microfocus X-ray tube according to the above operation principle has the following problems.
Since a fine converged electron beam collides with the target, a temperature rise concentrates adjacent an electron beam colliding spot on the target surface, thereby tending to evaporate the target material. The evaporation will result in the inconvenience of enlarging the X-ray focus or failing X-rays, which requires a maintenance operation such as a change of the X-ray tube or the target. When a powerful electron beam is emitted in order to increase X-ray dosage, the target material will evaporate momentarily to render the increase in X-ray dosage impossible.